Celestial Navigation: How Did Aviators “Shoot” the Sun and Stars?

Open and Exposed

On most early bombers and flying boats, such as this 1920s U.S. Navy Douglas PD-1, the navigator made sightings from the nose, where his view would not be obstructed by the biplane’s wings and struts. He had no protection from the elements. The force of the wind made his sextant difficult to handle.

TOOLS: Sextants and octants, astrocompasses, Sun compasses, astrographs, tables, line of position computers.ADVANTAGE: Works anywhere the sky can be seen.DISADVANTAGE: Requires clear line of sight to celestial bodies in the sky.CHALLENGES: Requires a high degree of skill or complex automated equipment.

First-generation aeronautical sextants had a bubble level to help the navigator locate the horizon in hazy or cloudy conditions. The instruments were hard to handle with air blowing past at 160 kilometers (100 miles) per hour.

Second-generation aeronautical sextants were much more aerodynamic, lightweight, and easy to handle. H. K. Beij of the National Bureau of Standards designed this one (and many others) for the U.S. Navy.

In 1927, the Army Air Corps Fokker C-2 Bird of Paradise became the first airplane to fly from the U.S. mainland to Hawaii. Navigator Albert Hegenberger had the new luxury of facing rear and behind a semi-protective windscreen while making his celestial sightings.

Before 1927, watches used with sextants for celestial sightings could only be set to the minute. A watch error of 30 seconds caused a navigational error of up to 12 kilometers (7 miles).
In 1927, P. V. H. Weems devised a watch with an adjustable second hand that could be set using radio time signals. These examples were his personal navigation watches.

Before 1927, watches used with sextants for celestial sightings could only be set to the minute. A watch error of 30 seconds caused a navigational error of up to 12 kilometers (7 miles).
In 1927, P. V. H. Weems devised a watch with an adjustable second hand that could be set using radio time signals. These examples were his personal navigation watches.

In the mid-1930s, the Longines-Wittnauer watch company marketed a line of watches designed in collaboration with Charles Lindbergh and P. V. H. Weems. The Hour Angle Watch sped computations for determining celestial lines of position. Its bezel and dial allowed navigators to read off the hour angle of a celestial object at Greenwich, eliminating a simple but troublesome calculation.

In 1932, Weems devised an almanac that greatly reduced the time needed for computing position based on celestial sightings. The U.S. Naval Observatory showed little interest in it at first, but the Royal Observatory in Greenwich began publishing it in time for World War II, when it proved invaluable.

This book of graphical solutions provided nighttime celestial calculations five times faster than other techniques. It required the sighting of Polaris and at least one other well-known navigational star.

A 1940s transport, such as the Douglas C-47 seen here, could fly more than 320 kilometers (200 miles) per hour and reach altitudes where the temperature fell well below freezing. Sticking one’s head out of the airplane to take sightings was no longer an option.

The astrodome was a technological marvel for its time. Not until the eve of World War II were manufacturers able to shape Plexiglas, a rugged transparent plastic, into a dome shape. The black disc at the top is a hanging point for a sextant, so navigators would not have to hold the heavy military models.

This 1938 mechanical computer was a remarkable attempt to automate complex navigational processes. Instead of spending minutes making manual calculations, navigators could simply input sextant observations and accurate time readings. It was too expensive and heavy for regular use. Only several dozen were made for the Air Corps and Navy.

The A-12 sextant was designed with Weems’ assistance just before World War II and manufactured by Ed Link, of Link trainer fame. It represented a new generation of “averaging” sextants that compensated for “Dutch roll” in airplanes by taking multiple sightings and computing an average without manual calculations.

The compact A-10 was one of the most commonly used sextants in the Army Air Forces. Tens of thousands were made during World War II, and many remained in service with the Air Force through the 1950s. Key features include a lighted bubble and a recording disk to determine averages.

The British-invented astrograph helped navigators determine the altitude curves of principal stars by projecting reels of film corresponding to certain latitudes. Suspended above the chart table in medium and heavy American bombers, the astrograph quickly fell out of favor because it was heavy and unreliable.

This averaging sextant was reliable and accurate but heavy. It saw wider use in the Navy than in the Army. The hook allowed the navigator to hang the bulky sextant from the top of an astrodome for more precise readings.

The astrocompass was mainly used to determine magnetic variation in the angular difference between an aircraft’s bearing to the magnetic North Pole and the geographic North Pole. Charles Blair used this one on the first trans-polar solo flight in 1951.

Navigators on civilian airliners and long-range military aircraft, such as the Lockheed RC-121D, began using retractable periscopic sextants in the late 1940s. They eliminated the risk of astrodome blowouts in pressurized aircraft and produced less drag. But their narrow field of view made finding a particular star more difficult.

Mounted behind the SR-71’s cockpit, this unit, affectionately known as “R2-D2,” computed navigational fixes using stars sighted through the lens in the top of the unit. These fixes were used to update the inertial navigation system and provided course guidance with an accuracy of at least 90 meters (300 feet). Some current aircraft and missile systems use improved versions as a backup to GPS.

"Black box" refers to more than just a flight data recorder for accident investigations. It often refers to electronic components in an aircraft that monitor or control flight or that support communication and navigation.

The Astroinertial Navigation System helped an SR-71 Blackbird crew set a coast-to-coast speed record. On March 6, 1990, while delivering the aircraft to the Museum, the crew flew across the United States in only 68 minutes. You can see that record-setting SR-71 at the Museum’s Steven F. Udvar-Hazy Center in Northern Virginia.